I'm renovating an older house in a cold climate (montana) and am trying to figure out the best / most economical way to insulate the upstairs ceiling space. The rooms have 4 foot "knee walls" which then intersect the ceiling/roof. I've got native 2x4 rafters, and not enough headroom to scab on extra framing to allow for more insulation space.
I know the best way to go would be to hire an insulation contractor to spray in foam, but the budget will not allow for this. What is my next best option?
My current thinking is to buy 3" rigid foam panels and tightly fit them between the rafters, leaving a 3/4" space between the top foam and underside of roof sheathing for venting. I'd like to use foil faced foam if I can find it for a little extra reflective help.
Does this approach make sense? Do I need the ventilation space? Is it worth paying extra for the foil facing? Is there another approach altogether that will give me a better R-value? Is there a certain type of rigid foam I should use?
Thanks for your help!